Traction control systems, also known as anti-slip regulation (ASR) systems, are used on vehicles to prevent loss of traction of the driven wheels so as to maintain control of the vehicle and to prevent deterioration of the accelerating performance due to longitudinal slip of the driving wheels. Traction control is necessary, for example, when excessive accelerator input is applied by the driver whilst driving on a road and the condition of the road surface is such that it is unable to cope with the torque applied.
Feedback control methods are typically employed in such systems, whereby the slip of the driving wheels (wheel slip) is monitored and, when it becomes excessive due to an excessive driving torque (e.g. sudden application of the accelerator pedal), appropriate action is taken to reduce the engine output and/or to apply a braking force to the driving wheels. For example a traction control system may signal an engine control unit (ECU) of the vehicle to reduce engine torque by retarding or suppressing the spark to one or more cylinders of the engine, reducing fuel supply to one or more of the engine cylinders, closing the throttle, or, in turbo-charged vehicles, actuating the boost control solenoid to reduce boost and therefore engine power. Additionally, the wheel brake to one or more of the wheels may be applied to control wheel slip.
Traction control systems are typically implemented in a vehicle as part of a stability control system (SCS) operable to enhance stability of a vehicle by detecting and reducing skidding. If a skid is detected whilst cornering, the stability control system is arranged automatically to apply braking to individual wheels to assist a driver in steering the vehicle in an intended direction, for example around a corner.
It is desirable to provide a vehicle having improved stability. This is particularly important in certain applications, for example in vehicles having electric propulsion motors. This is because electric propulsion motors are capable of delivering relatively high values of drive torque and relatively high rates of increase of drive torque.
It is also known to provide a control system for a motor vehicle for controlling one or more vehicle subsystems. U.S. Pat. No. 7,349,776, the content of which is hereby incorporated by reference, discloses a vehicle control system comprising a plurality of subsystem controllers including an engine management system, a transmission controller, a steering controller, a brakes controller and a suspension controller. The subsystem controllers are each operable in a plurality of subsystem function modes. The subsystem controllers are connected to a vehicle mode controller which controls the subsystem controllers to assume a required function mode so as to provide a number of driving modes for the vehicle. Each of the driving modes corresponds to a particular driving condition or set of driving conditions, and in each mode each of the sub-systems is set to the function mode most appropriate to those conditions. Such conditions are linked to types of terrain over which the vehicle may be driven such as grass/gravel/snow, mud and ruts, rock crawl, sand and a highway mode known as ‘special programs off’ (SPO). The vehicle mode controller may be referred to as a Terrain Response (TR)® System or controller.